1. Field of the Invention
The present invention relates to a control system for use on a rolling mill, and more particularly, to a control system and method in which the shifting distance and the shifting velocity of at least one working roll in the rolling mill are calculated and controlled by the requirements of both the shape of the strip being processed on the rolling mill plus the non-all statical friction condition on the contact surfaces of the strip and the rolls.
2. Summary of Related Art
The quality of cold-rolled thin-gage strip has increased considerably over the past few years in order to meet the quality demands of large users of strip metals, such as the automotive and appliance users. Strip shape is an important criterion for judging strip quality, and many techniques have been devised for controlling the strip shape during rolling mill production operations.
An ideal cold-rolled strip not only is to exhibit the same thickness over length and width, but also is to lie completely planar. Planeness should be preserved, even if the strip is cut into sections during further processing.
The requirements with respect to dimensional accuracy and planeness of a thin-gage strip have presented significant problems for steel and other metal processors. Certain flaws in planeness can be levelled by stretching, such as where the strip deviates uniformly from planeness in the width direction. Flaws in evenness that can be levelled by stretching are characterized in that they are generally delimited in one direction, i.e. in the longitudinal direction or in the transverse direction.
Deviations in planeness variable over the strip width and length are characterized by curved boundaries and cannot be stretched level by means of a simple bending process. In such case, non-uniform residual stress distributions are present in the longitudinal and transverse directions. The variable flaws appear as central and marginal wariness in the cold-rolled strip. The high requirements regarding the quality of rolled, thin-gage metal strip have resulted in increased interest in rolling mill systems and controls.
A rolling mill generally includes a supporting stand with at least two working rolls which bear on at least two back-up rolls. The rolls are carried at their two ends by means of rolling bearings, in chocks slidably mounted in the windows of the rolling mill supporting stand. Each chock typically includes two lateral guide faces sliding along corresponding sliding faces formed on the upright of the stand on either side of the chock.
A four-high rolling mill includes two working rolls each bearing on a back-up roll. A six-high rolling mill is provided with intermediate rolls between the back-up rolls and working rolls. In both cases, the axes of the rolls are place in the generally vertical gripping plane. Each working roll can also bear a larger number of intermediate and/or back-up rolls arranged symmetrically on either side of the gripping plane.
To obtain a uniform thickness in the direction transverse to the rolling direction, the bending or curving of the working rolls and, if appropriate, of the intermediate rolls, is carried out by means of bending devices acting on the chocks of the corresponding roll. The bending device for each chock generally consists of two sets of jacks arranged symmetrically on either side of the chock. Each bearing part of the chock bears on the two jacks set axially apart from one another symmetrically on either side of the mid-plane of the rolling bearing of the chock, so that the bending force is effectively distributed over the rolling bearings.
In four-high and six-high rolling mills, it is often advantageous to axially shift the rolls in order to achieve various objectives, including uniformity of the wear of the rolls and control of the planeness or profile of the metal strip. The edge of the strip causes wear on the surface of the roll during rolling, and the wear on the roll can be evened out by axially moving the roll.
The axial shifting of the rolls presents certain difficulties when the rolls are subject to a bending force. Consequently, the bending force and the axial shifting force are usually carried out separately, the bending force being stopped when the axial shifting takes place. During operation of the rolling mill, it is desirable to combine the effects of axial shifting the rolls and of bending the rolls. Consequently, it is also desirable to shift the rolls while continuing to bend the rolls.
A rolling mill for rolling metal strip uses small diameter working rolls. Since such working rolls have too small a diameter for application of the rolling torque directly to them, a number of multi-roll rolling mills have been developed in which the drive force is transmitted to the working rolls. The working rolls used to process the metal strip deflect between their oppositely held ends when the center portion of the roll is engaged by the metal strip. This deflection results in unacceptable product conditions due to its affect on the uniformity of the cross section and flatness of the strip and edge reduction.
Attempts have been made in the rolling mill industry to eliminate some of the adverse conditions in the rolling process. Vertical roll bending forces have been applied to the working rolls and the backup rolls. Special shaped working rolls and/or back up rolls, including fluid expandable rolls, have been used. Axial shifting of rolls has also been utilized to overcome the quality problems in production. In most cases, a combination of roll bending and roll shifting provides a reasonable solution to the irregular thickness problems of the metal strip.
The shifting of working rolls during production operations facilitates schedule-free rolling. Previously, working rolls were subject to unbalanced abrasion owing to the presence of the lateral edges of the rolled sheet material. This limited the number of same width strips which could be rolled consecutively. Operators often utilized a coffin schedule in which strips were rolled with the widths of the strips progressively narrowing.
In contrast, schedule free rolling permits rolling of any width strip by axially shifting the rolls to eliminate the unbalanced abrasion of the rolls. No limitations are placed on the order of selection of the widths of the strip. Strip products of the required width can be run based on product demand, and rolling mills can be included as in integral part of a production facility for producing the desired strip rolls.
U.S. Pat. No. 4,770,021 to Kobayashi et al. discloses a working roll shift type rolling mill which includes a shift device for shifting the working rolls in an axial direction and hydraulic cylinders for effecting a working roll bending pressure. Shift cylinders are used to shift the working rolls based on production time factors.
Axial shifting of working rolls in a rolling mill is also disclosed in U.S. Pat. Nos. 4,800,742 and 4,955,221 to Feldmann et al. The roll bodies are continuously curved over the entire length of the bodies. The shifting of the work rolls controls the shape of the gap between the two working rolls.
A control system is disclosed in U.S. Pat. No. 4,898,014 which controls the balance force exerted by a cylinder, which otherwise changes on the shifting of an associated roll. The control system includes a means for determining the amount of roll shifting, a second means, operatively associated with the first means, for determining the amount of change in the force imposed by the balance cylinder caused by the shifting, and a third means, responsive to the second means, for effecting a change in the balance force to maintain the force at a predetermined value.
U.S. Pat. No. 4,934,166 to Giacomoni discloses a rolling mill configuration which facilitates the simultaneous bending and axial shiftini of the work rolls. Each set of jacks furnished for the rolling mill bears in the direction of the bending force on a sliding piece mounted so as to slide vertically between two pairs of guide faces which are formed in a machined portion produced inside the supporting block. A sensor detects and measures the axial shifts, and sends a signal to a processor. The processor controls a servo valve to adjust the bending pressure of the working rolls.
In U.S. Pat. No. 5,174,144 to Kajiwara, a 4-high rolling mill is disclosed in which an equivalent crown amount can be increased and decreased to a desired value by an axial shift of the work rolls. The rolling mill includes a roll bending device for applying a bending force to the upper and lower work rolls, and a roll shift device for shifting the upper and lower work rolls in an axial direction.
One of the problems which occurs in axially shifting the work rolls is the scarring or scotch marks on the working rolls and the strip material. When a roll is shifted any distance in its axial direction, a friction force is produced in the contact zone of the roll surface. If the friction force is greater than its maximum statical friction force, the zone is in a slip state, which may produce scarring and scotch marks on the surface of the roll, which damages both the surface of the roll and the surface of the strip material. If the roll is damaged so that defects in the strip material occur during future rolling operations, the working roll will have to removed and repaired for future use. Any damage to the strip material presents a major problem and should be avoided. In addition to the scarring and scotch marks on the strip material, the friction force may even cause strip tearing, which creates production and product quality problems.
In an effort to improve overall quality of the strip being produced, rolling mill operators desire a control system to minimize or eliminate the scarring and scotch marks in the working rolls caused by axial shifting of the working rolls.